JACKSON, Mississippi (AP) — As the search continues for the missing Malaysian Airlines flight, a key question is what happened to the Boeing 777’s transponder.

Transponders are devices that emit electronic signals containing information that shows up on air traffic controllers’ screens. The information includes the plane’s unique identifying code and its direction, speed and altitude.

In the case of the Malaysian jetliner, the transponder stopped about an hour after takeoff.

Ross Aimer, an industry consultant and former airline pilot, says the 777 has two transponders, one a backup. If one goes out, the other comes on automatically. He says both can be shut off by turning a single knob on the cockpit panel.